In the mattress and bedding industry, it has long been known to use clips for attaching border wires to coil springs. These clips have a fairly standard generally U-shaped construction and are formed from a blank of metal material so that the clips have a crown portion and leg portions that depend from either end of the crown portion. One of the leg portions has a generally bifurcated construction to provide a pair of leg projections that are spaced from each other, and the other leg portion is formed into a single projection that is aligned with the space between the bifurcated leg projections so that the projections do not interfere with each other as they are clinched about adjacent wires to be attached.
For storage, transportation and application purposes, several different ways to collate the clips into an assembly of attached clips have been utilized. Generally, some sort of flexible connecting member is utilized so that the clips are identically oriented in end-to-end fashion in a row for being loaded in either a hand-held application tool or a vertical rotating clipping (VRC) machine which cut individual clips from the assembly and clinches them about wires to be held together.
Both the hand-held application tool and the VRC machine have a piston driven plunger which has a knife edge that cuts the particular connecting member utilized to hold the clips in assembled form. Various types of connectors have been employed for keeping the clips together in a flexible manner so that the clips can be compactly wound into a coil for storage and shipping purposes, and so that they can be unwound and fed into the application tool. Flexible wire connecting elements are commonly used to interconnect U-shaped clips. Parallel wires are attached to the clips by welding to the tops of the clip crowns and are severed by the aforementioned knife edge on the tool plunger. One shortcoming of the flexible wire connectors is that they cause a dulling of the application plunger tool knife edge which requires that the plunger blade be regularly sharpened or replaced so that the tool can consistently separate the clips from the collated assembly for clinching individual clips about the wires.
It is also known to hold clips together in assembly by plastic cords held in slots formed in the crown portion of the clips, see e.g., published application GB 2,023,216 A; U.S. Pat. No. 5,303,821; and U.S. Pat. No. 5,564,564. Plastic cords have the advantage over their metal wire counterparts of not having as much of a dulling effect on the knife edge of the application tool plunger, and not requiring that the blade edge be as sharp in order to sever the plastic cord. Because the plastic cords use slots formed in the clips as opposed to being welded directly to the clip material, the challenge with plastic cords is how best to form the slots and properly hold the cords therein. In this regard, manufacturing and material costs are important considerations, given the additional step required to form the necessary slots in the clips and the generally higher raw material costs for the plastic material versus metal wire.
While it is known to use a friction or press fit of plastic cords into the clip slots, this type of connection in and of itself normally is less than desirable for withstanding the various tensile forces and bending and twisting forces to which the attached cord will be subjected, such as occasioned by the coiling and uncoiling of the collating clip assembly, without separation of the clips from the cord. Also, because of cost considerations, it is desirable to use the smallest diameter cord possible so that the cord can be properly and easily severed by the application tool, while still maintaining the tolerances required for a consistent friction fit in the slot from clip to clip in the collated assembly. Another concern where very small diameter cords are used to be friction fit in clip slots is that the slots have to be also of a very small size which can cause problems when they are being formed. To form these small slots in a stamping or punching process, a relatively small sized punch is needed. Small punches that are not very robust may reduce punch life to a level that is not acceptable for high volume production of collated clip assemblies. Accordingly, the size of the plastic cord and thus of the slot in which it is friction fit is a compromise between a cord diameter that meets the desired cost criteria and allows for proper severing in the application tool, and an acceptable punch life for high volume clip assembly.
As previously discussed, there have been a number of prior clips which utilize special slot constructions to hold the plastic cord therein. It would be desirable to form the clip slots such that they hold the cord in the slots without requiring significant modifications to the manufacturing process. In most prior clips, their manufacture requires that additional clip material be bent or deformed for holding the cords in the slots of the clips, which generally calls for an additional manufacturing step, undesirably raising production costs. Accordingly, there is a need for a better formed clip slot for securely receiving plastic cords therein.
The small diameter plastic cords also tend to present a problem in keeping the cord at a circular cross-sectional shape within desired tolerances for being properly press fit in slots of the clips. With many plastic materials that meet the necessary cost criteria, there is a trade-off between achieving the desired cross-sectional shape and having the right tolerances for press fitting the cord into the slot.
Another factor in the design of the cord is the melt characteristics of the plastic material that is utilized. This can be important where the assembled mattress springs having the clips clinched thereon are tempered in a heat treat oven. If the heating is sufficient to melt the plastic of the cords in the clip slots, a problem arises if the plastic material beads together so as to create a bump over the top of the clips. Any small bumps like these are to be avoided, as they could create problems when the mattress material is applied over the assembled springs.